AbstractAISI 321 austenitic steel forms martensite due to quasi-static and cyclic loading. This presupposes the exceeding of the threshold value of cumulated plastic strain. This martensitic volume fraction or better its magnetic properties were investigated using HTC-SQUID technology. Several specimen batches were evaluated and thereby the load amplitudes and the test temperatures were varied (room temperature and 300°C). The experiments result in characteristic curves of the SQUID signals according to the fatigue damage which could be confirmed with comparative measurements with different methods, as e.g. ultrasonic absorption measurements. The extremely sensitive SQUID measuring technique allows also to detect information in specimens fatigued at a temperature of 300°C in which the phase fractions of strain-induced martensite are extraordinarily low.